1. Technical Field
The present invention relates generally to a flow rate sensor, and more particularly to a flow rate sensor which is suitable for determining the flow rate of intake air into an internal combustion engine, for example.
2. Background Art
A hot-wire air flow meter as a flow rate sensor is well known in the art. This meter is used for measuring the flow rate of intake air into an internal combustion engine. The meter generally includes a small winding type hot-wire probe which has a relatively great mechanical strength and an air temperature probe for detecting the temperature of intake air. These probes are disposed within a bypass air passage. It will be appreciated that a hot-wire air flow meter may provide a high degree of mechanical strength against the common usage conditions of automotive vehicles such as vibrations, thermal shock, and backfire. Miniaturization, ease of mass production, and rapid response time are also attributes of this type of sensor.
However, in such a hot-wire air flow meter, the hot-wire probe as a flow rate detecting element is arranged within a bypass air passage independent of the main air passage. Therefore, flow rate measurement is subject to inaccuracy due to pulsations caused by the opening and closing of the intake valves of the engine which causes flow velocity and distribution to vary continually, creating an extremely insecure condition with asymmetrical distribution therein due to the many curved sections and necessary straight sections within an air passage. Due to spatial restrictions a hot wire type flow rate sensor cannot take such conditions into account.
In view of the above drawbacks, a uniquely designed flow rate sensor provided with honeycomb shaped flow rate detecting elements has been developed. This sensor is disclosed in U.S. patent application No. 352,317 field on May 15, 1989, entitled "FLOW RATE SENSOR" by Hiroshi KOBAYASHI, and assigned to NISSAN MOTOR CO., LTD.
This type of the flow rate sensor includes a plurality of cells defined by the flow rate detecting elements which are made of metallic foil and a circular support member which covers the detecting elements to retain it in a flow passage. A fluid flowing within the flow passage passes through the cells without pressure drop.
However, the mechanical strength of this flow rate sensor is insufficient to be used in extremely insecure environments such as an intake manifold to which vibrations are transmitted from an engine. Further, to form the honeycomb structure of the above-cited sensor, complex processes are necessary wherein an insulating layer and an electrical resistance layer are laminated on a metallic foil by a spattering method, increasing its manufacturing cost.